A drilling fluid is circulated from an earthen surface down through a drilled out wellbore to a drilling face and back to the surface when drilling into a subterranean formation in pursuit of economically recoverable hydrocarbons. Drilling fluids are specifically designed to perform a number of functions, including cooling and lubricating the drill bit, removing drill cuttings from the wellbore, supporting a part of the weight of the drill pipe and drill bit, providing a hydrostatic head to maintain the integrity of the wellbore walls and control the well, preventing significant flow of fluids across the wellbore face into the wellbore and vice versa, and minimizing or eliminating damage to the reservoir.
A common problem encountered during drilling operations is “lost circulation”, which is the excessive flow of drilling fluids out of the wellbore. The drilling fluids are either lost to the formation matrix or to voids in direct communication with the wellbore. Lost circulation is undesirable from an economic standpoint because it requires one to continually replenish the wellbore with costly drilling fluid. Lost circulation is also undesirable from an operational and safety standpoint because it can destabilize permeable formations and damage the pay zone, and in extreme cases it can result in a blowout of the hydrocarbon zone followed by a well fire.
After drilling, the well construction operations move into a “completion” phase wherein the drilling fluid may continue to be used in the well or it may be displaced and a completion fluid or completion brine emplaced to take over some of the roles of the drilling fluid, especially well control, while enabling other functions for which the drilling fluid may not be ideally suited. The well control aspect described above, of course, must be addressed continuously to avoid destabilizing permeable formations, damaging the pay zone, and leading in extreme cases to a blowout of the hydrocarbon zone followed by a well fire. During completion operations, the need to avoid losses of fluid during circulation continues, but it also broadens to include the need to avoid excessive fluid loss during periods in which fluids are no longer circulated. The term used to cover this broadened functionality is “fluid loss control” but it should be emphasized that fluid loss control and lost circulation control are very similar technologies. Therefore, the teachings in accordance with the present invention can be utilized in fluid loss control during completions in much the same fashion and using much the same materials as in lost circulation during drilling.
Drilling fluids are typically formulated to contain one or more viscosifiers, such as polymeric additives, which, among other things, can help to control lost circulation. Often specific substances are administered—either as additional components of the drilling fluid or as distinct fluids—to inhibit the flow of drilling fluids from the wellbore into the formation; these are termed “lost circulation materials” or “lost circulation treatments”.
Unfortunately many lost circulation treatments known in the art are unsatisfactory because of operational limitations restricting their utility. For example, some lost circulation treatments are ineffective in the presence of high salt concentration brines. Others undergo thermal degradation when subjected to high operational temperatures. The most significant shortcoming of conventional lost circulation treatments is their inability to effectively control lost circulation encountered when drilling through voids occurring in the formation.
Thus, a need exists for a lost circulation treatment using a material that effectively prevents or reduces lost circulation of drilling fluids, particularly in cases where conventional lost circulation materials are ineffective. Further, a need exists for a lost circulation treatment which has sufficient strength and integrity to minimize lost circulation into voids in direct communication with the wellbore, such as fractures, fracture networks, vugs, washouts, cavities, and the like.